1. Field of the Invention
The present invention relates to apparatus and methods for estimating and compensating sampling clock offset (SCO) for orthogonal frequency division multiplexing (OFDM) communications, and more particularly to the apparatus and the SCO estimation and compensation methods for multi-band OFDM-based ultra-wideband (UWB) systems.
2. Description of Related Art
Multi-band orthogonal frequency division multiplexing (OFDM) based ultra-wideband (UWB) communication has attracted considerable attention in the recent years, as described in the following two references: [1] “A. Batra, J. Balakrishnan, G. R. Aiello, J. R. Foerster, and A. Dabak, “Design of a multiband OFDM system for realistic UWB channel environments,” IEEE Trans. Microwave Theory and Techniques, vol. 52, no. 9, pp. 2123-2138, September 2004.”; [2] “WiMedia MBOA, MultiBand OFDM Physical Layer Specification, ver. 1.1.5, Jul. 14, 2006.”. The large bandwidth occupancy of UWB (from 3.1 GHz to 10.6 GHz) and the high efficiency in spectrum utilization provided by OFDM make it possible for the OFDM-UWB technology to achieve very high channel capacity. The OFDM-UWB system can provide low-cost and high-speed wireless connectivity among devices within a short range. The wireless universal serial bus (USB), for example, has adopted the OFDM-UWB radio layer with the data rate up to 480 Mbps.
The extremely wide-band processing has brought a lot of challenges to the OFDM-UWB system design, especially to the design of some crucial receiving modules such as the symbol timing, the carrier frequency offset (CFO) and sampling clock offset (SCO) compensation, as well as the channel frequency response (CFR) estimation. The SCO issue is caused by the sampling clock frequency mismatch between the transmitter and the receiver. Since the UWB device operates at a very high sampling rate (at least 528 Mbps), a small SCO shall cause the phase-shift among the received frequency-domain complex data at all subcarriers, which, after accumulating over a certain period, becomes significant and will seriously degrade the system performance if not well tracked and compensated. Due to its high complexity, the maximum likelihood (ML) phase tracking approach is prohibitive in this case, as described in references [3] “P.-Y. Tsai, H.-Y. Kang, and T.-D. Chiueh, “Joint weighted least-squares estimation of carrier-frequency offset and timing offset for OFDM systems over multipath fading channels,” IEEE Trans. Veh. Technol., vol. 54, no. 1, pp. 211-223, January 2005.” and [4] “J. Liu and J. Li, “Parameter estimation and error reduction for OFDM-based WLANs,” IEEE Trans. Mobile. Computing, vol. 3, no. 2, pp. 152-163, April-June 2004”. Moreover, a time-domain interpolator is used to compensate the SCO in reference [3]. However, the time-domain interpolator is implementation expensive in case of high-speed processing.
Therefore, efficient SCO estimation and compensation technologies are critically desirable for improving the performance of the multi-band OFDM-based UWB system.